In wavelength division multiplexing (WDM) and dense wavelength division multiplexing (DWDM) optical networks many optical signals at different frequencies, i.e., having different wavelengths, are multiplexed together for transmission over a single medium. Note that, as frequency and wavelength are inversely related, they are often used interchangeably to indicate the same thing by those skilled in the art. The same may be done herein. In order to avoid crosstalk between the signals at the various frequencies, i.e., interchannel crosstalk, it is important that the carrier for each signal be properly locked to its designated frequency. Typically the different frequencies employed in a system are defined by a prescribed frequency set which typically enumerates the frequency values to be used or the spacing between them. This set or spacing is often expressed as a grid. For example, the International Telecommunications Union (ITU) has defined several standardized grids, e.g. for DWDM networks, and these grids define optical channels having carrier separations of 100 GHz, 50 GHz, and even as small as 25 GHz. Such densely multiplexed optical carriers need to be well locked to their designated, i.e., desired or target, optical frequency.
Frequency locking to the ITU grid is usually achieved using a reference Fabry-Perot etalon in combination with complicated temperature measurement and adjustment circuits in order to keep the resonances of the etalon fixed to the ITU grid. The etalon is usually made of quartz glass because of its small thermo-optic and linear thermal expansion coefficients.
Large scale photonic integrated circuits (PIC) appear to have promise for realizing cost and energy efficient WDM and DWDM transmitters. However, the construction of absolute wavelength references on a chip is challenging because of the large thermo-optic coefficient of both III-V and Si materials, and thus doing so requires employing advanced temperature stabilization techniques, which is undesirable. Integration of the conventional quartz glass made Fabry-Perot etalon on a chip is also not attractive because of the relatively large dimensions of the etalon. Moreover, any interferometer or resonant filter fabricated on such a photonic chip, e.g., one made of silicon, will suffer from fabrication errors in that it has been found that most of the time the resulting actually fabricated filter shape is significantly different from the one anticipated in the design process.
Generally, prior art methods of on-chip wavelength locking can be classified into two groups.
In the first group of solutions, the temperature of the chip/cavity is measured and through use of a closed loop control circuit the bias voltage/current on the intra-cavity components is adjusted with the help of look-up tables. One disadvantage of such a solution is that the devices are biased with a large DC power to allow adjustment in both the negative and positive directions depending on whether the temperature of the chip increases or decreases.
In the second group of solutions, the temperature of the chip is kept constant with the help of temperature sensors and thermoelectrical coolers. This solution controls the whole transmitter chip and does not correct temperature gradients on the chip. It also suffers from relatively large delay times because of the large dimension of the chip.
Note that in both of the above groups of solutions the temperature change itself is used as information about the wavelength drift and that no absolute wavelength measurements are performed. Also, in the second group of solutions the wavelength is measured and locked to the grid by means of an external Fabry-Perot etalon. This solution, even though practical and reliable, is disadvantageous in that 1) it is not compact and cost effective, 2) it is sensitive to mechanical vibrations, and 3) it still requires active control of the temperature.